1. Field of the Invention
The invention proceeds from a device for measuring the density of a gas, in particular of the insulating gas of a gas-insulated electrical installation, comprising two resonators each containing a piezoelectric crystal and each containing at least two electrodes applied to the crystal, of which two resonators a first is mounted in a first chamber containing gas to be measured and a second is mounted in a second chamber sealed with respect to the gas. The invention furthermore proceeds from methods of producing a gas-density measuring device of this type.
2. Discussion of Background
A device of the type mentioned in the introduction is described in EP-A-0 484 569. In the case of this device, the density of sulfur hexafluoride in a gas-insulated electrical high-voltage installation is measured by means of two resonators. For identical external conditions, i.e. at a predetermined temperature and with the same environment, such as, in particular, vacuum, both resonators have identical resonance frequencies. One of the two resonators serves as sensor and is disposed in a chamber containing sulfur hexafluoride. The gas-density-dependent detuning of its resonance frequency is in this case a measure of the density of the sulfur hexafluoride to be measured. Another of the two resonators is provided for reference purposes and is preferably disposed under vacuum. A measured signal which is proportional to the density of the sulfur hexafluoride is tapped off in an evaluating electronic system to which the output signals of the two resonators are applied.
In this case, piezoelectrically excited tuning-fork crystals are used as resonators. These crystals are cut out of a quartz wafer in large numbers in the clock industry in the production of timepieces and are each coated with two gold electrodes. These gold electrodes serve, on the one hand, for the piezoelectric excitation of the tuning-fork crystal with the aid of an electrical oscillatory circuit. On the other hand, they also serve, however, to tune the individual tuning-fork crystals to a standard fundamental resonance frequency of 32,768 Hz at a temperature of 25.degree. C.